1. Field of the Invention
This invention relates, in general, to work holders for electrical circuit assemblages or wiring systems and, more specifically, to printed circuit board holders useful during the application of solder paste and components to the boards.
2. Description of the Prior Art
When manufacturing printed circuit boards (PCB's) for use in electronic equipment, the bare boards are frequently screened with a solder paste or cream to deposit the paste where the component leads will be soldered to the board. The paste is selectively applied only to the solder areas by the configuration or design of the screen. Once screened, a PCB is transported to another station or device which places the components onto the board at the proper locations. Next, the board with the components in place is transported to a soldering station. While these processes can be performed on single PCB's, one at a time, it is often desirable in high speed production to perform some or all of these processes on several boards at substantially the same time.
Work board holders have been used, according to the prior art, to hold or secure a plurality of boards in predetermined positions so that the various operations can be performed simultaneously on the boards. According to usual prior art practice, separate work board holders are used for paste screening and for component placement, since the physical requirements of the holders are different. For the screening process, the holder must secure the boards with clamps, clips, or other devices which do not appreciably extend above the surface of the board. Since the paste is applied with a smooth blade which wipes or moves across the screen and board, objects extending above the board can interfere with paste application and reduce the life of the screen and/or blade. However, the protrusion of locking tabs or clamps above the board surface is not of critical concern during component placement. Thus, it is conventional practice to use more positive and secure holding devices during component placement than when the boards are screened.
A prior art method of processing PCB's involved the use of a fixture having several openings or slots for placement of PCB's. The boards were held in place for the screening application of solder paste by thin clips which extended for a short distance over the surface of the boards. After screening, the boards were removed from that fixture and inserted into another fixture which firmly held the boards in place with locking tabs which extended above the surface of the boards. Even though this arrangement has been useful in processing several PCB's at a time, a considerable amount of time is needed to transfer the boards from the one holder or fixture to the other. In addition, the clips holding the boards during the screening operation, even though thin, can still cause difficulties with the screen member and with the life of the flux application blade.
Therefore, it is desirable, and an object of this invention, to provide a universal work board holder which is useful in screening and component placement operations during assembly operations involving printed circuit boards.